Assessing Impacts of Irrigation on Streamflow

Abstract

A method for assessing the impacts on streamflow resulting from withdrawing water from the stream channel or from shallow wells adjacent to the stream for irrigation is presented. Assessment is based on a detailed study at a base station on the stream. Monthly irrigation demands are estimated and added to measured streamflow to determine the natural streamflow. The natural streamflow is correlated with flow on a nearby watershed to develop an extensive series of monthly flows. The procedure produces probability distributions for average monthly flow for each month of the growing season, for different levels of irrigation usage. The flow distributions include the original variation and the additional variation produced by the irrigation withdrawal. The streamflow information developed can be combined with biological or water quality models to assess the impacts of reduced flow on instream biology, water quality, and treatment requirements of waste discharged to streams.